This invention is directed to the problem of minimizing the production of sludge, particularly sludges relatively more toxic than municipal domestic sludge, by using an autothermal aerobic digestion process to efficiently convert biodegradable organic matter, in wastewater streams, to harmless carbon dioxide and water, with aerobic, thermophilic, living microorganisms employed in a substantially autothermal aerobic ("ATA") process in which an autothermal aerobic bioreactor ("ATAB") and a membrane means, together referred to as an ATA membrane bioreactor ("ATA MBR") are used. Such living microorganisms are generally referred to hereinafter, as "cells" because they are essentially unicellular, and the term "cells" is convenient and brief. The acronym "ATA" is used herein specifically to identify the type of process disclosed herein, so as not to confuse it with a prior art process disclosed in U.S. Pat. No. 4,915,840, issued to Rozich et al. and referred to as an "ATAD" process. The essential feature of the operation of the ATA MBR is that it is "substantially autothermal", that is, no more than 20% of the heat required to maintain the reaction is provided by an external source, preferably less than 10%, and most preferably none. The biochemical oxidation reaction, by itself, generates sufficient heat to maintain a temperature at which the cells thrive.
The '840 process also uses an autothermal bioreactor, but in addition, requires a hydrolyzer significantly to reduce the amount of sludge generated.
More specifically, this invention relates to purifying any wastewater stream suitable for treatment with (i) thermophilic cells which thrive in an environment at a temperature in the range from 40.degree. C. to about 75.degree. C., more preferably from 55.degree.-75.degree. C., and, (ii) caldo-active cells which thrive in an environment at a temperature in the range from 80.degree. C. to about 85.degree. C., or even higher (see New Scientist, Vol. 137, 1854, 1993). In the range from 75.degree. C. to 80.degree. C. the cells may be said to be both thermophilic and/or caldo-active.
For brevity and convenience, thermophilic and/or caldo-active cells are referred to herein, simply as "hot cells" because each has been especially acclimated to function effectively as a "hot" activated sludge, and the ATA MBR as a "hot" system, as opposed to a mesophilic aerobic bioreactor ("MPB") in combination with a membrane means referred to as a MPB MBR "cool" system. Properly acclimated hot cells will ingest even the recalcitrant compounds in a "waste fluid" stream which they then come to regard as a substantially completely biodegradable "feed". The particular hot cells chosen for use in this "hot" process depends upon the % chemical oxidation demand (% COD) removal desired, the rate at which the removal is to be effected, and other factors.
The dominant characteristic of hot cells is that, under the controlled conditions of the process of this invention, they grow and replicate ("log-growth phase") only slightly more rapidly than they "self-destruct" or are "self-consumed", that is, consumed by other cells of the same species in a "log-death phase", as will be explained below. This process is therefore intolerant to the loss of as little as 10% of the number of live cells in the ATAB at any time during its operation. Remembering that most of the biomass in the mixed liquor consists of dead cells, it is critical that one maintain the required total suspended solids (TSS) in the ATAB. An unacceptable loss of live cells occurs when either (i) more than 10% of the "mixed liquor" from the ATAB is lost, or (ii) 10% more of the live cells are consumed than there are living cells regenerated, that is the "death-rate" is greater than 10%. This requirement relating to guarding against loss of live hot cells from an ATAB is referred to herein as the "essentially no-loss requirement". It is equally important that the rate at which living cells are regenerated be restricted to no more than 10% greater than the rate at which they are consumed, because this results in generation of biomass rather than its destruction. This restriction is particularly applicable when an ATAB is operating at the low end of its temperature range (about 45.degree. C.). The critical "essentially no loss" requirement is usually of greater concern than over-reproduction because there is less than a 10% net generation of live cells in a ATAD reactor operating in the range from 55.degree. C.-75.degree. C., and the higher the temperature range of operation the slower the rate of reproduction. For successful operation of the ATAB the population of living cells, that is CFU/ml (colony-forming units/ml), is stabilized within.+-.10% of an optimal determined by routine trial and error such as one skilled in the art undertakes, thus meeting the criticality of the "stable living cell population" requirement. Depending upon the nature of the feed and the temperature of operation, the CFU/ml in the ATAB may range from 10.sup.6 or lower, up to 10.sup.12 or above, equivalent to a concentration of 5 to 25 gm biomass per liter, the number of live cells being difficult to determine. "Mixed liquor" refers to activated sludge biomass including cells, dead and alive. In the ATAB, thermophilic cells are especially acclimated to the task they are to perform in the elevated temperature, "hot" environment of the ATA biooxidation zone. Note however, that thermophilic cells may metabolize in a reactor which is not autothermal.
The "essentially no-loss requirement" is met by combining an ultrafiltration (UF) and/or microfiltration (MF) means with a thermophilic or caldo-active "hot" bioreactor. The term "hot bioreactor" as used herein refers to use of "hot" cells in a MBR in which organic matter in wastewater is biochemically oxidized. With a "hot" ATAB, the lower limit at which this ATA process operates is dictated by the biological oxygen demand ("BOD") of the feed. The BOD is too low when the ATAB cannot operate substantially autothermally. All references herein to BOD refer to "theoretical or ultimate BOD", meaning that the BOD is measured using duly acclimated cells, after digestion for an infinitely long time at 60.degree. C., which in essence, is a measure of all biodegradable material present. Preferably a feed has a COD loading in the range from 4-50 kg/m.sup.3 /day (24 hr), preferably 10-30 kg/m.sup.3 /day, and the ATA MBR delivers an effluent of permeate which is free of TSS. It has been found that the feed COD may be increased as the COD loading decreases in the bioreactor due to cell growth, provided of course, at least enough molecular oxygen as is required by the growing cells, is delivered to the mixed liquor in the ATAB.
The basic technology, using a MBR was disclosed a quarter of a century ago in U.S. Pat. No. 3,472,765 to Budd et al. They suggested a combination using a UF or reverse osmosis ("RO") membrane, not only to avoid the time penalty of using gravity settling technology to separate sludge from mixed liquor, but also to provide essentially solids-free reusable and sewerable ("permeate") water to be recovered. The solids-containing stream ("concentrate") from the membrane separator was recycled to the bioreactor, and eventually the undegraded high molecular weight materials and salts were purged.
The essential process characteristic of the '765 patent was the requirement that a constant volume of mixed liquor is maintained in the reactor ("constant reactor volume") by varying the flow of feed. This choice was made with the recognition that a MBR for readily biodegradable sludge, is preferably run at constant relatively low hydraulic retention time ("HRT") in the range from 20 min-30 min. Specifically in the '765 patent, the aim was to convert municipal sewage by generating biomass deemed to be a valuable byproduct, not to destroy it. This limitation of constant HRT requires that both, the flow to the reaction vessel and, the volume of mixed liquor in it, be maintained constant.
The '765 process chose to maintain the reactor volume constant by varying the flow. This choice is unexpectedly inapt because, in practice, varying the flow causes the short HRT to vary too widely. The ATA process requires that the HRT be essentially constant in each of two different operating embodiments; (1) the first, in which the flow rate of feed to the reactor is held essentially constant, and, essentially all concentrate and a minor portion, from about 0.1% to less than 50% of the permeate, is recycled to the ATAB; and (2) the second, in which essentially all concentrate and no portion of the permeate is recycled to the ATAB. Thus, neither embodiment maintains an essentially constant reactor volume by varying the feed flow. By "essentially constant" is meant.+-.10%, preferably.+-.5%. Details of operation for each of the two embodiments are presented hereunder.
The flux of a membrane system declines as a function of operating time from an initial `high` value, to a value about 50% of the high value, and progressively gets lower until the membrane is cleaned. To accommodate constant flow to the reactor, a membrane filtration system is sized for the minimum flux, just before cleaning (`worst case flux`). This operating requirement results in excess capacity of the membrane system at all times except when the membrane is fouled. Since the level in the reactor would soon decrease if the entire permeate is disposed of, the flow of permeate is modulated. Such modulation of permeate flow may be done by one of two, first and second, methods; or, first by one (the first) method, then with the other (the second) method, depending upon the nature of the membrane used, as is described in the detailed description below.
In either embodiment of the process, it is essential for effective utilization of organic matter by hot cells, that large biodegradable solids, too large to be easily metabolized by the cells, be comminuted to finely divided solids in the size range from about 90-180 .mu.m (US Standard Test Sieve Series Wire Cloth in the range from No. 80 to No. 170) or smaller. Preferably, essentially no solids greater than about 106 .mu.m (U.S. Standard Test Sieve Series Wire Cloth No. 140) enter the reactor. The only solids other than the hot cells in the mixed liquor, are the aforementioned finely divided organic solids, and the products generated in the mixed liquor.
In `design` examples the '840 process shows only a marginal reduction in sludge with the implementation of the hydrolysis assist (compare Tables 1 and 2). In each case, the '840 process requires that feed to its ATAD reactor be essentially all biomass (cells) generated by pretreatment of incoming wastewater in a MPB. In operation for its design purpose, a portion of that biomass is prehydrolyzed to provide nutrition for the cells in the thermophilic reactor, so that feed to the ATAD reactor is a combination of the biomass from the MPB and the prehydrolyzed biomass. Further, the effluent from the ATAD zone must be flowed to the MPB for further biooxidation, and to generate a more settlable sludge than is discharged from the ATAD reactor. Since no membrane separator is used with either reactor, a purified effluent can be discharged only after effluent from the MPB is settled.
As will presently be evident, a single ATA MBR (used in a "single stage process") allows the novel ATA process of this invention to discharge a purified effluent, free of all suspended solids, without further biooxidation, directly after mixed liquor is filtered in a MF or UF membrane filtration zone.
Typically, wastewater streams most economically treated in this ATA process are generated in industrial/chemical and agricultural manufacturing facilities, referred to herein as "chemicals wastewater" and "agricultural wastewater" respectively, though relatively easily biodegradable wastewater in local municipalities, referred to herein as "municipal" or "domestic" wastewater for brevity and convenience, may also be treated. By "relatively easily biodegradable wastewater" is meant a feed having a BOD in the range from about 100-10,000 mg/L, which requires an HRT of less than 24 hr (referred to herein as "HRT24") in a conventional MPB operating at 10.degree. C.-30.degree. C. with mixed liquor having volatile suspended solids (VSS) in the range from 1000-5000 mg/L. Since an MPB typically treats a feed with an ultimate BOD of less than 1,000 mg/L, the HRT is typically less than 8 hr. The rate of flow of such wastewater streams typically varies from one hour to the next, and daily rates may vary by as much as an order of magnitude (tenfold) or more.
Because "chemicals wastewater" and "agricultural wastewater" streams may contain a mixture of "recalcitrant" compounds and "refractory" compounds, not typically present in "municipal" wastewater, hereafter, the former (one or more of the combination of chemicals, and industrial and agricultural wastewaters) will be referred to as "waste fluid" in those instances where the emphasis is on their atypical content, and to distinguish them from "municipal" wastewater. Because the term "wastewater feed" or "feed" for brevity, is used herein to refer to the entire volume of a liquid feedstream to this ATA process, whether domestic wastewater or waste fluid, all waste fluid is "feed", but not all "feed" is waste fluid.
An ATA MBR is typically used (single stage embodiment), for "feed" containing low BOD loadings below 5000 mg/L when the feed is available at a temperature near the operating temperature of the ATAB, and for high BOD loadings above 5,000 mg/L, with a wide variety of "chemicals" including C.sub.6 -C.sub.14 esters, carbohydrates and fatty acids, which will often include some domestic sewage, the proportion of which sewage relative to the concentration of chemicals, varies daily. A chemicals "waste fluid" stream may contain small quantities of "recalcitrant", "highly recalcitrant" and "refractory", compounds and typically does. Chemicals in waste fluids may include recalcitrant or very recalcitrant aliphatic, cycloaliphatic and aromatic hydrocarbons, and numerous oxygen-containing derivatives thereof. It may be required to treat recalcitrant and highly recalcitrant compounds at a COD of about 5000 mg/L to yield a permeate with essentially no COD (&lt;100 mg/L); but if mixed with wastewater containing relatively easily biodegraded organic material which provides at least 10% of the COD loading of the reactor, a COD above 10,000 mg/L in the feed may yield a comparable permeate.
By a "recalcitrant" compound is meant one which would not be significantly biodegraded in a typical conventional municipal water treatment/activated sludge facility operating with a relatively short HRT, &lt;24 hr. Many recalcitrant compounds, however, are biodegraded by cells duly acclimated to those compounds with a reasonably short HRT less than 5 days, often as little as 1 day, even at a relatively high COD loading of 5 kg/m.sup.3 /day. An agricultural waste fluid may contain simple and complex carbohydrate waste, muscle tissue, hair, and C.sub.4 -C.sub.24 branched chain fatty acids, both saturated and unsaturated, each of which may be a recalcitrant compound. A chemicals waste fluid may contain compounds such as chlorinated hydrocarbons with varying content, by weight, of chlorine in each molecule of chlorinated hydrocarbon, such as a stream produced in a vinyl chloride plant; synthetic esters and oils such as are used in metalworking fluids discharged from a metal-working facility; tannery and Abbatoir wastes; wastewater containing manufactured organic solvents and other compounds, and the like.
By a "highly recalcitrant" compound is meant one which is biodegraded with difficulty even by cells which are duly acclimated to such a compound, that is, the compound is eventually biodegraded by cells acclimated to the compound, but requires a HRT in the range from about 5 to 10 days at the same COD loading of 5 kg/m.sup.3 /day used before. Finally, "refractory" compounds are those which will not be biodegraded by cells acclimated to recalcitrant and highly recalcitrant compounds, irrespective of any realistic length of HRT and COD. As one would expect, mineral impurities such as silica, oxides of numerous metals, and the like are not biodegradable. In a different category from minerals, inorganic compounds such as metal salts and numerous organic compounds which are highly toxic to animal and plant life are also deemed "refractory". Such compounds include the metal sulfates, thiosulfates, chlorides, etc. and highly halogenated compounds such as chlorinated aromatic, aliphatic and cycloaliphatic compounds, e.g. hexachlorobenzene, halogenated long C.sub.12 -C.sub.16 branched chain esters of various fatty acids, and, trichlorophenols.
Even a relatively small content of such recalcitrant and highly recalcitrant compounds in the range from 10 ppm-100 ppm, requires that cells be used which are duly acclimated to their task, or those compounds will not be degraded.
A particular example of a waste fluid stream (a particular stream referred to herein is an ester-containing aqueous stream) is one which is produced in a chemical plant for the manufacture of dialkyl phthalates by the esterification of phthalic arthydride with mixtures of n-octyl and n-decyl alcohol, and a relatively smaller amount of n-hexyl alcohol than either of the other alcohols. In such a stream, the esters are formed in a wide range, from di-C.sub.6 -alkyl to di-C.sub.13 -alkyl esters, all of which do not have the same degree of biodegradability. For this reason, in addition to testing a combined stream of various laboratory-grade esters which were mixed to simulate a composition which might be present in such a waste fluid, several individual esters were also tested.
A particular ester-containing aqueous stream, like most "feeds" suitable for this ATA process, contains a predominant amount of biodegradable organic matter, the ratio of theoretical or ultimate BOD to COD usually being &gt;0.6 (greater than 0.6), preferably in the range from 0.8 to 1.0, which is the range for domestic wastewater containing domestic sewage.
The precise organic content of any feed will depend upon its source, just as even the components and flow of a domestic sewage stream may vary. Streams other than domestic, vary not only in flow rate and the components, but also in temperature and the concentration of their individual components (referred to as "chemicals" in a waste fluid).
The problem of sludge disposal from an ATAB is addressed herein as being specifically directed to the biochemical conversion of chemicals and agricultural waste fluids containing from 1,000 ppm-200,000 ppm (parts per million, or mg/L) preferably from 10,000-100,000 mg/L of biodegradable but recalcitrant organic compounds. Such a high level of biodegradable compounds are not typically present in domestic waste.
It is known that waste fluids containing a relatively low level of biodegradable compounds, in the range from 100-1000 ppm, cannot normally be treated either effectively or economically, in a thermophilic process. Surprisingly however, waste fluids may benefit from being treated in an ATAB if they are first converted to a biomass of sufficiently high BOD.
Particular examples of aqueous chemicals waste fluids suitable as feed to an ATAB, are those containing (i) a mixture of synthetic metal-working fluids, fats oils and greases ("FOG") mixed with synthetic organic and organometallic compounds, some of which are insoluble solids, others emulsifiable liquids, and still others soluble solids and liquids; (ii) C.sub.3 -C.sub.26 alcohols, fatty acids, alkaryl and aralkyl carboxylic acids and esters thereof, particularly alcohols and lower C.sub.1 -C.sub.6 alkyl esters of fatty acids, or, C.sub.6 -C.sub.13 alkyl esters of C.sub.6 -C.sub.13 alcohols, or, di(C.sub.6 -C.sub.13)alkyl phthalic acid esters from a plant making plasticizers; (iii) monocyclic monoolefins such as cyclohexene, monocyclic diolefines such as cyclopentadiene, polycyclic cycloolefins, particularly a mixture of mono- and di-olefinically unsaturated polycyclic olefins, such as norbornene, methyl norbornene, tetracyclododecene, and dicyclo-pentadiene, from a polymer plant; (iv) straight chain C.sub.8 -C.sub.20 paraffins from a lubricant plant; (iv) solvents and chemicals used in the electronics industry for making, etching and washing printed circuit boards and microprocessors; etc.
Particular examples of aqueous agricultural waste fluids contain (i) cheese whey from a cheese making plant; (ii) dispersed blood, hair, proteinaceous tissue and fat from meat packing plants; (iii) dispersed particles of grain flour from cereal processing plants; (iv) fermentation broths from ethanol plants; (v) milk and cream from a dairy, or ice cream plant, etc.
Domestic sewage, and waste fluids, when biochemically converted in a conventional activated sludge process, produce a substantial amount of sludge because of the conditions of degradation. The goal of this process is to demonstrate that, in a thermophilic process known to minimize sludge production, an ATA MBR not only nurtures and maintains a high concentration of hot cells which are duly acclimated to their designated task, but also prevents hot cells from being lost. In such a process, if necessary, domestic sewage and many wastewater streams may be biodegraded with the production of essentially no sludge. This is of particular interest under conditions where neither disposal of sludge nor its storage are desirable alternatives, as for example, in urban areas with limited landfill space, or on board a ship or other marine vessel which may carry several hundred persons, or more.
The constituents of a typical feed to the ATA process vary in biodegradability across the full spectrum of difficulty. Most preferably, feeds to be treated contain high levels of biodegradable organic matter, e.g. feeds such as cheese whey.
A typical toxic chemical waste fluid treated in a ATAB contains aromatic and aliphatic and alkylaromatic hydrocarbons, some of which may be halogenated, sulfonated, oxygenated, etc. along with domestic sewage. The waste fluid often has an insignificant ammonia-nitrogen content in the range from 10 ppm to 50 ppm. Such waste fluids contain a minimal amount of refractory (non-biodegradable) matter, and are received as feed for this process directly after the feed components are collected from locations at which they originate, and without any preliminary treatment if it contains an insignificant amount of large solids &gt;180 .mu.m, preferably, substantially none greater than 106 .mu.m.
"Feed" to the ATA reaction zone has a relatively high ultimate BOD&gt;5,000 mg/L, preferably in the range from 10,000 to 250,000 mg/L; and, most preferably from 25,000-100,000 mg/L. Upon treatment in the system, an effluent permeate from the membrane means typically has a COD of &lt;1000 mg/L, and no measurable suspended solids, all of which are filtered out by a microporous or semipermeable membrane.
It will now be evident that, though a typical domestic sewage having a BOD to COD ratio &gt;0.8 g/L may be "feed" for an ATA MBR used by itself ("single stage"), because the feed contains easily biodegradable organic matter, it is uneconomical to supply the heat required to operate the reactor above about 45.degree. C. unless exigent restrictions exist relating to the disposal or storage of sludge.
Acclimated "hot" cells used in the ATA process can degrade at least 90%, and often 100% of the organic matter in the "feed", if the feed is free of refractory compounds and the solids retention time (SRT) and HRT are long enough. In the novel ATA process described herein, the COD is essentially equal to the BOD, because there is so little normally refractory matter; or, what normally is non-biodegradable matter (and is therefore measured only as COD), is now consumed in the ATA process by the hot cells, as nutrients (carbon source). A typical proteinaceous and hydrocarbon "feed" to the ATA process includes hydrocarbons and oxyhydrocarbons with a COD in the range from 10,000-150,000 mg/L.
More specifically, since the primary goal of this process is to convert predominantly recalcitrant contaminants in the feed to products such as carbon dioxide and water by biochemical oxidation, in a single stage bioreactor with a minimal amount of sludge production, the use of any type of adsorbent or separation medium (together referred to as "media") is clearly contraindicated. The goal required the development of a single stage solely biological reaction zone in a more cost-effective process than any currently known; and, to develop a specific process in which an ATA MBR would generate only a minimal amount of sludge to be disposed of, most preferably none. It transpired that such a single stage ATA MBR may be combined with a MP MBR to provide a combination of functions which proves to be unexpectedly effective and economical.
As is well known, a MPB (mesophilic bioreactor, not a MP MBR) which is effective in a conventional suspension process, is wholly incapable of producing minimal sludge if it is to produce a reasonably "clean" effluent, unless it is operated under "extended aeration" condition. Such conditions, except under the most unusual circumstances, are uneconomical. Despite the known efficacy of thermophilic reactors (not a MBR), biomass or activated sludges produced therein do not settle either as easily, or as fast as a conventional mesophilic activated sludge, and such deficient settling properties of thermophilic sludge cannot be successfully dealt with in a conventional clarifier. Therefore, the effluent from a conventional ATAB is treated with acid or base to give the effluent a hydrolytic assist before returning the effluent to the MPB. See Proceedings, 40th Annual Purdue Industrial Waste Conference, pp. 775-784 Ann Arbor Press, Ann Arbor, Mich. (1986).
Still further, the received wisdom in the art was that membranes were prone to heavy fouling under continuous usage conditions, and moreover were more susceptible to damage in the temperature range in which an autothermal bioreactor operates. The '765 Budd et al teaching is non-enabling because there is no indication of what particular membranes, composite or not, might prove useful; or, how to avoid fouling of those membranes at a throughput which provides a reasonable yield of permeate; nor is there any indication as to what class (UF, MF or RO), or type (channel, spiral, tubular or hollow fiber), or material (polysulfone, fluoropolymer, etc) of membrane will actually provide a practical separation. Though the relative flow rate of concentrate recycled to the reactor is stated to be 20 times the feed to the reactor, there is no suggestion how to cope with the damage which may be done under inapt process conditions which a too-high back pressure against a composite membrane may separate it from its support. It is essential that permeate pressure never exceed the pressure on the feed, even within the module. In particular, there is no suggestion that a high enough velocity of mixed liquor through the robes of a tubular membrane module might be achieved by controlling the pressure drop through the tubes so as simultaneously to lower the transmembrane pressure and keep the membrane wall from fouling. For this reason, in all the years since the '765 patent issued, those skilled in the art failed to appreciate the potential of either a MF or UF membrane in the service suggested by Budd et al.
Endogenous respiration or auto-oxidation (or "autoxidation") of the mixed liquor is the result of the cells becoming self-consuming to some degree. This occurs in the aerobic treatment of organic waste in an aqueous stream contacted with an ample supply of dissolved oxygen ("DO"), preferably introduced as oxygen-enriched air, or oxygen alone, and in the absence of readily available organic nutrient. By "ample supply" we refer to a DO concentration of at least 1 ppm, preferably from 3 to 10 ppm in a reactor under ambient pressure.
This process requires the controlled autoxidation of mixed liquor such that hot cells are generated no more than 10% faster than they are consumed. Under such conditions there is a reduction in the net rate of sludge production relative to digestion in a conventional MPB.
It is not likely that one skilled in the an can predict the effects of a combination of normally toxic components of a recalcitrant waste fluid on the rate of metabolization of the living cells, particularly to the extent they may be consumed in a thermophilic environment. Nor could one predict at what level of COD such a waste fluid would allow the acclimated cells to generate an autothermal reaction assuming the cells could have a satisfactory uptake of oxygen.
Extended aeration frequently is used to reduce the net production of sludge because increased HRT increases the amount of autoxidation. As one would expect, however, such extended aeration results in an uneconomical increase in the size of tanks to provide the requisite HRT to 24 hr or more. An HRT of 18 hr in a MPB is far more time than needed for BOD assimilation of a typical feed. Larger tanks, the costs of agitation of a large volume of aqueous medium, and, also of aerating the tanks, add up to an uneconomic processing cost/unit volume of feed treated.
The promise of minimizing sludge production, along with several other benefits of a thermophilic biochemical oxidation process, appeared to be negated by the high cost of the equipment necessary and the costs of operating that equipment. To help evaluate the problem, an extensive study of the numerous considerations relating to the evaluation, development and operation of such a system was presented in a paper titled "Autothermal Thermophilic Aerobic Digestion in Federal Republic of Germany", at the 40th Annual Purdue Industrial Waste Conference, Lafayette, Ind. in May 14-16, 1985, and in U.S. Pat. No. 4,246,099 to Gould et al.
Another determinative factor for successful operation of this process, is the preferred operation of the ATA reactor at constant volume. Such operation resulted in specifying the system so as to generate permeate at a critical rate greater than that at which permeate is recycled to the reactor. This high rate of permeate production makes it possible to provide a permeate recycle, if desired, in addition to the recycle of concentrate to the ATAB. Despite the apparent contradiction of recycling the permeate after going to the trouble of generating it, a flow rate of permeate recycle, less than the flow rate of incoming raw feed, is desirable for the reactor to operate at constant volume, and to feed a membranous UF zone at a constant rate of flow. The reason for the permeate recycle is explained in detail herebelow. Alternatively, permeate flow may be continuously controlled to match the in-flow of feed so that there is no recycle of permeate, as explained below.
This process incorporates the foregoing features and relies upon the unique operation, of an ambient pressure, aerobic thermophilic reactor, most preferably in combination with an equalization tank and a membrane device tailored to separate permeate and molecules of predetermined size which are products of biodegradation, from biomass which is recycled.
As will readily be evident, the '840 process relies upon at least the following: the two reactors (one thermophilic, the other mesophilic), a thickening tank, a hydrolyzer tank, and two gravity-settling tanks. A first settling step is the routine removal of heavy solids, easily accomplished in a primary clarifier which removes insoluble organics and grit. The effluent from this clarifier, containing mainly dissolved organic solids, is then biochemically convened to biomass in a conventional activated sludge reactor (referred to in the '840 patent as an "aerobic zone", but referred to in this specification as a "mesophilic aeration zone" since the thermophilic zone is also "aerobic"). The conventional sludge effluent from the '840 aerobic zone is then settled in a secondary clarifier.
What is not stated is that effluent from the ATAD tank 34 would contain duly acclimated cells which would be lost when the effluent 35 is flowed to the aerobic zone 6. The ATAD fails to meet the "essentially no-loss requirement" imposed by conditions of temperature in the upper portion of the range from 40.degree. C.-75.degree. C. Further, if the suspended solids in the ATAD effluent are finely divided, that is, less than 180 .mu.m, it is impractical to thicken the effluent in a settling tank or centrifuge, or other conventional thickening means such as the use of flocculants and thickening chemicals.
A most important feature of the ATAD '840 process is that the treated effluent from the ATAD bioreactor must be further biochemically oxidized in the aerobic zone before the effluent from the latter may be clarified and discharged as purified water. To accomplish this task requires all the equipment identified above, not to mention the many high-service-duty pumps (for high flow rates required), several large tanks and the extensive network of large diameter piping connecting the tanks. Clearly, the '840 patent provides an equivocally effective solution to the problem.
The ATA process of this invention provides a far more effective, unexpectedly simpler and more economic solution, than any proffered in the prior art, provided it (ATA process) is operated within a relatively narrowly defined window of operating criteria. Our process does away with the conventional aerobic reaction zone as well as the hydrolyzer, thickener, primary and secondary clarifiers. An appropriate membrane device, in combination with a single stage ATAB and associated pump and piping means, is all that is basically required if a feed, essentially free of large suspended solids &gt;180 .mu.m and too large to be degraded by the thermophilic cells, is to be treated in a desirable dilution. Feed is diluted to provide enough water to prevent the build-up of salts and non-biodegradable byproducts to toxic or inhibitory levels for the biomass.
It is such a process which was tested with a variety of synthetic dialkyl phthalate esters which simulate those genetically actually present in a plasticizer plant, and for which esters, singly and in combination, the experimental data set forth herein were obtained. No attempt was made to add the corresponding aliphatic alcohols to the ester-contaminated feed which was formulated to simulate the biodegradability of the actual feed to be treated, because it is known that hot cells which can biodegrade the alkaryl esters can easily biodegrade the aliphatic alcohols.
Though each mechanical component in the system is known, the combination used in the single stage ATA process is found to be economically effective only if operated substantially autothermally, as described hereunder to treat delivered feed which contains a very high concentration, at least 1% TSS, of biodegradable organic matter. The ATAB is operated to maintain a predetermined concentration of recalcitrant compounds and more easily degraded organic matter as TSS, and the membrane device is operated as a MF or UF tubular membrane at low pressure, in the range from about 170 to 1035 Kpa (10 psig to 135 psig), preferably under 50 psig (450 kPa). Such operation results in a controlled, high mass flow of solids-containing concentrate as a recycle stream.
The mass flow from the ATAB is surprisingly high, yet (i) for recalcitrant compounds requiring an HRT&gt;24 hr in a MPB, the mass flow provides a long enough SRT to degrade the organic waste in delivered feed, and also (ii) completes biodegradation of the recalcitrant organic matter with a HRT of less than 10 days, and preferably less than 48 hr. The key to providing the foregoing is to use a feed with a high ultimate BOD content, &gt;0.6 times its COD, and to maintain a stable population of living cells at ambient pressure in the ATAB.